There have been worldwide endeavors for some years to synthesize so-called "intrinsically" electroconductive polymers. These are taken to mean polymer materials which have an inherent conductivity without the addition of electroconductive substances such as metal powders or fibers, conductive black or the like. Examples of such polymers are polyacetylene, polypyrrole, polythiophene, polyaniline, polyparaphenylene, polyphenylene sulfide, etc. However, polyconjugated bonding systems of this type are only electroconductive in the doped state, i.e. they must be converted into a conductive state by means of an oxidant or reducing agent in an electrochemical or chemical reaction. However, all of the materials listed above are insoluble and infusible in the doped state i.e. are unsuitable for further processing.
Therefore, up to a few years ago, there were few concrete applications for intrinsically electroconductive polymers. A further disadvantage was the low stability of the novel materials, in particular in moist atmospheres.
In order to obtain processable electroconductive polymers, soluble, intrinsically electroconductive polymers were developed (cf. R. L. Elsenbaumer, K. Y. Jen and R. Oboodi, Synth. Met. 15 (1986), 169). In particular, doped polyalkoxythiophenes synthesized by electrochemical methods are distinguished by high stability (M. Feldhues et al., Synth. Met. 28 (1989), C487). In doped form, these materials are soluble in low percentages in organic aprotic solvents, such as toluene, THF, acetonitrile, diethylformamide and N-methylpyrrolidone, and are therefore suitable as base materials for the electroconductive and/or antistatic coating of substrates (EP-A-0 328 981, EP-A-0 257 573 and EP-A-0 328 982).
However, it has proven very disadvantageous in the production of such coatings on an industrial scale that organic solvents have to be used, since many organic solvents today require industrial post-combustion, which is very expensive and complex. Attempts are therefore increasingly being made, in particular for the continuous coating of flexible substrates, such as, for example, films, to employ aqueous systems in which at least a large part of the organic solvent has been replaced by water.
In principle, two methods are conceivable for obtaining aqueous coating formulations based on intrinsically electroconductive polymers. The first is the development of novel, water-soluble, conductive polymers (A. O. Patil et al., J. Am. Chem. Soc. 109 (1987), 1858; E. E. Havinga et al., Polymer Bulletin 18 (1987), 277; A. O. Patil et al., Synth. Met. 20 (1987), 151). A second method is the preparation of aqueous dispersions. However, industrial processing of the materials, which became known as a result of the new development of water-soluble polymers, has hitherto been unsuccessful due to only very low electrical conductivity values achieved of below 0.1 S/cm and due to poor stability in water.
S. P. Armes and M. Aldissi, Polymer 31 (1990), 569, describe the preparation of an aqueous polypyrrole dispersion. In the process described, a steric stabilizer (poly-2-vinylpyridylbutyl methacrylate) must be added in order to avoid particle agglomeration of the polypyrrole, which can only be dispersed with difficulty. The specific conductivity of the dried powder is from 1 to 2 S/cm. No suitability of the material for coating purposes has been disclosed.
S. P. Armes et al., J. Coll. Interf. Sci. 118 (1987), 410, likewise describes an aqueous polypyrrole dispersion, in which the dispersion stabilizer employed is polyvinyl acetate. The individual conductive particles (not the liquid dispersion) exhibit a specific conductivity of 5 S/cm at a particle diameter of from 100 to 150 nm. The properties of thin layers made from these dispersions are not described.
DE-A-38 34 526 describes the coating of substrates with free-flowing dispersions based on polyaniline. It is also possible to use aqueous media here. It is disadvantageous that, after coating is complete and the solvent has been removed, it is necessary to convert the layer chemically or electrochemically into the conductive form necessary for its later function.
In both cases, insoluble, intrinsically conductive polymers in the doped form are used. Aqueous dispersions of conductive polymers which are soluble in organic solvents have not been disclosed.